TESS Finds Potentially Habitable Super-Earth 31 Light-Years Away

After completing its first year of observations in the southern sky, NASA's Transiting Exoplanet Survey Satellite has spotted some intriguing new exoplanets only 31 light-years away from Earth.

Multiple exoplanets -- planets orbiting stars outside our solar system -- were discovered orbiting an M-dwarf star, called GJ 357 in the Hydra constellation. The star is 40% cooler than our sun and only about a third of the sun's mass and size. A study describing the three planets was published this week in the journal Astronomy & Astrophysics.

The first exoplanet discovered around the star was GJ 357 b.

The exoplanet is 22% larger and 80% more massive than Earth, making it a super-Earth. It's 11 times closer to its star than Mercury is to our sun and the researchers estimate that it has an average temperature of 490 degrees Fahrenheit. This does not account for any potential warming effects of an atmosphere if it has one. It completes one orbit around the star every 3.9 days.

"We describe GJ 357 b as a 'hot Earth,'" said Enric Pallé, study co-author and astrophysicist at the Institute of Astrophysics of the Canary Islands. "Although it cannot host life, it is noteworthy as the third-nearest transiting exoplanet known to date and one of the best rocky planets we have for measuring the composition of any atmosphere it may possess."

The researchers also discovered more signals of exoplanets in the system.

GJ 357 d, a super-Earth that is 6.1 times the Earth's mass, is the most intriguing because it orbits the star at a distance where the temperature might be just right to support liquid water on the surface.

"GJ 357 d is located within the outer edge of its star's habitable zone, where it receives about the same amount of stellar energy from its star as Mars does from the Sun," said Diana Kossakowski, study co-author at at the Max Planck Institute for Astronomy. "If the planet has a dense atmosphere, which will take future studies to determine, it could trap enough heat to warm the planet and allow liquid water on its surface."

The researchers don't know if the super-Earth is rocky like our own planet, but it orbits the star every 55.7 days and has a temperature of negative 64 degrees Fahrenheit. An atmosphere could cause it to be warmer.

"This is exciting, as this is humanity's first nearby super-Earth that could harbor life -- uncovered with help from TESS, our small, mighty mission with a huge reach," said Lisa Kaltenegger, study author, associate professor of astronomy and director of Cornell's Carl Sagan Institute. "With a thick atmosphere, the planet GJ 357 d could maintain liquid water on its surface like Earth and we could pick out signs of life with upcoming telescopes soon to be online."

In the middle of those two planets is GJ 357 c. It's 3.4 times the mass of Earth and zips around the planet every 9.1 days, reaching a temperature of 260 degrees Fahrenheit.

"In a way, these planets were hiding in measurements made at numerous observatories over many years," said Rafael Luque, study author and doctoral student at the Canary Islands institute, who led the discovery team. "It took TESS to point us to an interesting star where we could uncover them."